Wire rod having enhanced strength and impact toughness and preparation method for same

ABSTRACT

Provided is a wire rod having enhanced strength and impact toughness comprising, by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur (5):0.020% or less, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%, nitrogen (N): 0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as a residual component thereof, and other unavoidable impurities. A microstructure includes bainitic ferrite in an area fraction of 90% or more, and a martensite/austenite (M/A) constituent as a residual component thereof.

TECHNICAL FIELD

The present disclosure relates to a wire rod having enhanced strengthand impact toughness and a preparation method for the same. The wire rodmay be used for components of industrial machines, vehicles, and thelike, exposed to various external load environments.

BACKGROUND ART

Recently, efforts to reduce emissions of carbon dioxide, a main cause ofenvironmental pollution, have become a global issue. In line with this,active movements to regulate vehicle exhaust gas emissions exist. As ameasure to comply with such regulations, automakers are attempting toreduce emissions through improvement of fuel efficiency. However, inorder to improve fuel efficiency, vehicles are required to belightweight while having high performance. Thus, the requirement forhigh strength in materials for vehicles and components thereof isincreased. In addition, since demand for resistance to the shock ofexternal impacts is also increased, impact resistance is also recognizedas an important material property of a material or a component.

A wire rod having a ferrite or perlite structure is limited in terms ofsecuring excellent strength and impact toughness. In a material having astructure described above according to the related art, impact toughnessis high, but strength is relatively low. When cold drawing is performedin order to increase strength, high strength may be obtained. However,there may be a disadvantage in that impact toughness may rapidlydecrease in proportion to an increase in strength.

Thus, in general, in order to simultaneously realize excellent strengthand impact toughness, a bainite structure or a tempered martensitestructure are used. A bainite structure may be obtained through aconstant temperature transformation heat treatment using a steelmaterial having been hot rolled, and a tempered martensite structure maybe obtained through a quenching and tempering heat treatment. However,since there are limitations on obtaining the structures described abovethrough only using hot rolling and continuous cooling processesaccording to the related art, it is necessary to perform an additionalheat treatment process described above using a hot rolled steelmaterial.

If high strength and excellent impact toughness are secured without anadditional heat treatment being performed, a portion of a process from amaterial to a component having been manufacturing may be omitted or maybe simplified. Thus, there is an advantage in which productivity isimproved and manufacturing costs are lowered.

However, a wire rod, capable of being provided with a stable bainitic ormartensitic structure using hot rolling and continuous cooling processeswithout an additional heat treatment process being required, has not yetbeen developed, so demand for the development of such a wire rod hasemerged.

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a wire rod having highstrength and excellent impact toughness with hot rolling and continuouscooling processes without an additional heat treatment process, and apreparation method for same.

An objective to be solved in the present disclosure is not limited tothe above-mentioned objective, and other objectives not mentioned can beclearly understood by those skilled in the art from the followingdescription.

Technical Solution

According to an aspect of the present disclosure, a wire rod havingenhanced strength and impact toughness includes, by wt %: carbon (C):0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0% to4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron(B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%, nitrogen (N):0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as aresidual component thereof, and other unavoidable impurities, wherein amicrostructure includes bainitic ferrite in an area fraction of 90% ormore, and a martensite/austenite (M/A) constituent as a residualcomponent thereof.

According to another aspect of the present disclosure, a method ofpreparing a wire rod having enhanced strength and impact toughnessincludes: reheating a steel material including, by wt %, carbon (C):0.05% to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 3.0% to4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020% or less, boron(B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%, nitrogen (N):0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron (Fe) as aresidual component thereof, and other unavoidable impurities; hotrolling the steel material having been reheated; cooling the steelmaterial at a rate of 0.1° C./s to 2° C./s in a temperature within arange of Bf° C. to Bf−50° C., after the hot rolling; and air cooling thesteel material having been cooled.

Advantageous Effects

According to an exemplary embodiment in the present disclosure, onlyusing hot rolling and continuous cooling processes, a wire rod havingenhanced strength and impact toughness, required for a material and acomponent for an industrial machine or a vehicle, may be provided.

In addition, an additional heat treatment process according to therelated art may be omitted, whereby there is an advantageous in reducingtotal manufacturing costs.

BEST MODE FOR INVENTION

Hereinafter, the present disclosure will be described in detail.

First, a wire rod according to the present disclosure will be describedin detail. The wire rod according to the present disclosure includes, bywt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese(Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020%or less, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%,nitrogen (N): 0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron(Fe) as a residual component thereof, and other unavoidable impurities.

Hereinafter, a steel component of a wire rod and a limitation reason ofa composition range will be described in detail (hereinafter, wt %).

Carbon (C): 0.05% to 0.15%

Carbon is an essential element for ensuring strength, and is dissolvedin steel or exists in the form of carbide or cementite. The simplestmethod for increasing strength is to form carbide or cementite byincreasing the content of carbon. However, ductility and impacttoughness are reduced in this case. Thus, it is required to adjust anaddition amount of carbon within a certain range. In the presentdisclosure, it is preferable to add the content of C in the range of0.05% to 0.15%. In a case in which the content of carbon is less than0.05%, it may be difficult to achieve target strength. In a case inwhich the content of carbon exceeds 0.15%, impact toughness may besignificantly reduced.

Silicon (Si): 0.2% or Less

Silicon is known as a deoxidizing element, along with aluminum, and isan element for improving strength. Silicon is known as an element highlyeffective in increasing strength through solid solution strengthening ofa steel material through being dissolved in ferrite when added to thesteel material. However, when silicon is added thereto, strength may besignificantly increased, but ductility and impact toughness aresignificantly reduced. In a case of a cold forging component requiringsufficient ductility, the addition of silicon should be limited. In thepresent disclosure, to secure excellent impact toughness whilesignificantly reducing a decrease in strength, a content of silicon of0.2% or less is included. In a case in which the content of siliconexceeds 0.2%, there may be limitations on securing target impacttoughness. More preferably, a content of silicon of 0.1% or less isincluded.

Manganese (Mn): 3.0% to 4.0%

Manganese allows strength of a steel material to be increased andhardenability thereof to be improved, thereby allowing a low temperaturestructure such as bainite or martensite to be easily formed in a widerange of cooling rates. However, in a case in which the content ofmanganese is lower than 3.0%, hardenability is insufficient, and thus,it is difficult to stably secure a low temperature structure in acontinuous cooling process after hot rolling. In addition, in a case inwhich the content of manganese exceeds 4.0%, hardenability issignificantly high, and thus, such a case is inappropriate since amartensite structure may be obtained even in air cooling. In thisregard, in the present disclosure, it is preferable to include a contentof manganese of 3.0% to 4.0%.

Phosphorus (P): 0.020% or Less

Phosphorus is a main cause of a reduction in toughness and a reductionin delayed fracture resistance, as being segregated in a grain boundary,and thus, it is preferable not to include phosphorous. For this reason,an upper limit thereof in the present disclosure is limited to 0.020%.

Sulfur (S): 0.020% or Less

Sulfur is segregated in a grain boundary to reduce toughness and allowsa low melting point emulsion to be formed so as to inhibit hot rolling,and thus, it is preferable not to include sulfur. For this reason, anupper limit thereof in the present disclosure is limited to 0.020%.

Boron (B): 0.0010% to 0.0030%

Boron is an element for improving hardenability, and is an elementsuppressing formation of ferrite in cooling through being diffused at anaustenite grain boundary, and allowing bainite or martensite to beeasily formed. However, in a case in which an addition amount thereof isless than 0.0010%, it is difficult to expect an effect due to addition.In a case in which an addition amount thereof exceeds 0.0030%, it isdifficult to expect an increase in an effect, while grain boundarystrength is reduced due to the precipitation of boron-based nitride in agrain boundary, thereby decreasing hot workability. Thus, in thisregard, a range of addition of boron in the present disclosure is0.0010% to 0.0030%.

Titanium (Ti): 0.010% to 0.030%

Titanium has the highest reactivity with nitrogen, thereby formingnitride first. When most of nitrogen in steel is exhausted by formingtitanium nitride (TiN) due to the addition of titanium, titanium allowsboron to exist in a soluble state by preventing precipitation of BN,thereby obtaining an effect of improving hardenability. However, in acase in which an addition amount thereof is less than 0.010%, an effectdue to addition is insufficient. In a case in which an addition amountthereof exceeds 0.030%, coarse nitride is formed, thereby reducingmechanical properties. In this regard, the content of titanium in thepresent disclosure is 0.010% to 0.030%.

Nitrogen (N): 0.0050% or Less

Nitrogen is maintained in a state of being soluble with boron. Tosufficiently exhibit an effect of improving hardenability, it ispreferable not include nitrogen. Thus, in the present disclosure, it ispreferable that the content thereof be 0.0050% or less.

Aluminum (Al): 0.010% to 0.050%

Aluminum is a powerful deoxidizing element, and allows oxygen in steelto be removed so as to improve cleanliness and is combined with nitrogendissolved in steel so as to form aluminum nitride (AlN), therebyimproving impact toughness. In the present disclosure, aluminum isactively added. In a case in which a content thereof is less than0.010%, it is difficult to expect an addition effect thereof. In a casein which a content thereof exceeds 0.050%, a large amount of aluminainclusion is generated, thereby significantly reducing mechanicalproperties. In this regard, in the present disclosure, it is preferablethat the content of aluminum be in the range of 0.010% to 0.050%.

In addition to compositions described above, chromium (Cr) of 0.3% lessthan may be additionally included. Chromium increases strength andhardenability of a steel material, in a manner similar to manganese. Ina case in which the content of chromium is 0.3% or more, hardenabilitymay be improved and strength may be increased due to a solid solutionstrengthening effect, but impact toughness may be reduced. In thisregard, in the present disclosure, it is preferable to include thecontent of chromium in the range of less than 0.3%.

In addition to composition described above, a residual component thereofincludes Fe and unavoidable impurities. In the present disclosure,addition of other alloys in addition to an alloy composition describedabove is not excluded.

Meanwhile, in the present disclosure, it is preferable that the contentof manganese (Mn), titanium (Ti), boron (B), and nitrogen (N) iscontained to satisfy Relational Expression 1,

Mn+5(Ti−3.5N)/B≧5.0.   [Relational Expression 1]

Here, in Relational Expression 1, manganese (Mn), titanium (Ti), boron(B), and nitrogen (N) refer to the contents by weight of elements,respectively.

In the present disclosure, manganese increases hardenability, even whena cooling rate is relatively low, manganese allows bainitic ferrite tobe easily generated. In addition, titanium is combined with nitrogen toform a nitride and allows boron to be sufficiently dissolved in steel,thereby suppressing the generation of ferrite and allowing bainiticferrite to be easily generated.

The inventors of the present disclosure have repeatedly conductedresearch and experiments based on the description above. As a result,when the relationship among manganese, titanium, boron, and nitrogensatisfies Mn+5(Ti−3.5N)/B≧5.0 based on weight %, it is recognized that awire rod having a bainitic ferrite structure with more excellentstrength and impact toughness is provided and Relational Expression 1 isderived.

In addition, in the present disclosure, it is preferable that thecontent of manganese (Mn) and silicon (Si) satisfies RelationalExpression 2,

Mn/Si≧18.   [Relational Expression 2]

Here, in Relational Expression 2, manganese (Mn) and silicon (Si) referto the contents by weight of elements, respectively.

In the present disclosure, manganese increases hardenability. Thus, evenwhen a cooling rate is relatively low, manganese allows bainite to beeasily generated. In addition, silicon is dissolved in steel, and thus,strength may be increased, whereas impact toughness may be reduced.

The inventors have repeatedly conducted research and experiments basedon the description above. As a result, when the relationship betweenmanganese and silicon satisfies Mn/Si≧18 based on weight o, it isconfirmed that a wire rod having a bainitic ferrite structure with moreexcellent strength and impact toughness is provided, and thus, aRelational Expression of a composition component is proposed.

Meanwhile, it is preferable that a wire rod according to the presentdisclosure be provided with an arbitrary cross sectional area in which aratio of a maximum concentration [Mn_(max)] and a minimum concentration[Mn_(min)] of manganese satisfies Relational Expression 3,

[Mn_(max)]/[Mn_(min)]≦3.   [Relational Expression 3]

In the present disclosure, manganese increases hardenability. Even whena cooling rate is relatively low, manganese allows bainitic ferrite tobe easily generated. However, when manganese is locally segregated,martensite may be easily generated. In addition, in an area in whichmanganese is depleted, ferrite may be formed. Thus, a microstructure maybe non-uniform, and impact toughness may be reduced.

The inventors have repeatedly conducted researches and experiments basedon the description above. As a result, when a ratio of a maximumconcentration and a minimum concentration of manganese is 3 or less inan arbitrary cross sectional area of the wire rod, it is confirmed thata wire rod having a bainitic ferrite structure with excellent strengthand impact toughness is provided, and thus, Relational Expression isproposed.

Hereinafter, a microstructure according to the present disclosure willbe described in detail.

It is preferable that a microstructure of a wire rod according to thepresent disclosure includes bainitic ferrite of 90 area % or more and aresidual martensite-austenite (M/A) constituent. Meanwhile, bainite maybe referred to as various terms depending to the content of carbon ormorphology. According to the related art, bainite is referred to asupper/lower bainite above the range of medium carbon (about 0.2 wt % to0.45 wt %). However, within the range of low carbon of 0.2% or less,bainite is referred to as bainitic ferrite, acicular ferrite, granularferrite, or the like, depending on a temperature range. In the presentdisclosure, due to a low carbon region, a bainitic ferrite structure isincluded.

Since a microstructure of a wire rod according to the present disclosureincludes bainitic ferrite of 90 area % or more, excellent strength andimpact toughness may be secured. When a phase fraction, not of bainiticferrite, but of ferrite according to the related art is increased, itmay be advantageous in terms of impact toughness. However, since it islimited to preventing strength from being reduced, it is not preferable.

Meanwhile, the martensite-austenite constituent is formed along abainitic ferrite grain boundary which is columnar. When a fractionthereof is high, strength of a steel material may be increased. However,since impact toughness may be degraded, it is preferable to manage afraction thereof to be as low as possible. In this regard, in thepresent disclosure, it is preferable that a fraction of themartensite-austenite constituent is managed to be, by area %, 10% orless, (that is, a bainitic ferrite structure, which is columnar, of 90%or more). To obtain a microstructure of a wire rod according to thepresent disclosure described above, in the present disclosure, after asteel material is hot rolled, a cooling end temperature and a coolingrate when cooling the steel material are adjusted, thereby effectivelyachieving obtainment.

Meanwhile, it is preferable that a grain size of themartensite/austenite (M/A) constituent be 5 μm or less. When a grainsize of the martensite/austenite (M/A) constituent exceeds 5 μm, an areaof an interface in contact with a bainitic ferrite base is increased,impact toughness may be reduced.

Next, a method of manufacturing a wire rod according to the presentdisclosure will be described in detail.

A method of manufacturing a wire rod according to the present disclosuremay include: reheating steel having a composition described above afterpreparing the steel; hot rolling a steel material having been reheated;cooling the steel material at a rate of 0.1° C./s to 2° C./s to atemperature within a range of Bf° C. to Bf−50° C. after the hot rolling;and air cooling the steel material having been cooled.

First, in the present disclosure, after a steel material having acomposition component described above is prepared, the steel material isreheated. A reheating temperature applied in the present disclosure ispreferably in the range of 1000° C. to 1100° C.

A form of the steel material is not particularly limited, but it ispreferably a bloom or a billet according to the related art.

Next, the steel material having been reheated is hot rolled tomanufacture a wire rod. A finish hot rolling temperature of the hotrolling is not particularly limited, but it is preferable to be in therange of 850° C. to 950° C.

The steel material having been hot rolled is cooled, and it ispreferable that cooling is performed at a cooling rate of 0.1° C./s to2° C./s to a temperature within a range of Bf° C. to Bf−50° C. When acooling end temperature exceeds Bf, it is difficult to secure asufficient amount of a bainitic ferrite structure. When the cooling endtemperature is less than Bf−50° C., a steel material is sufficientlycooled to be easily handled. However, since productivity is lowered, itis preferable that the cooling end temperature be in a temperaturewithin a range of Bf° C. to Bf−50° C. Bf refers to a temperature inwhich phase transformation from austenite to bainite or bainitic ferriteends.

In the present disclosure, since a bainitic ferrite structure is securedby performing continuous cooling after hot rolling, excellent strengthand impact toughness may be secured. Here, since a heat treatment suchas quenching and tempering, performed according to the related art, maybe omitted, an additional process is not required. Thus, it isadvantageous in terms of manufacturing costs.

In addition, in the present disclosure, it is preferable that a sectionfrom a cooling start temperature to a cooling end temperature is cooledat a cooling rate of 0.1° C./s to 2° C./s. When the cooling rate is lessthan 0.1° C./s, formation of pro-eutectoid ferrite increases. When thecooling rate exceeds 2° C./s, formation of martensite increases. Thus,strength and impact toughness may be reduced. In the present disclosure,it is preferable that the cooling rate be managed to 0.1° C./s to 2°C./s.

As described above, as a cooling rate is secured in a cooling section, awire rod having enhanced strength and impact toughness, having bainiticferrite of 90% or more by area fraction may be obtained.

[Mode for Invention]

Hereinafter, an exemplary embodiment according to the present disclosurewill be described in detail. The exemplary embodiment described belowaccording to the present disclosure is provided for the purpose ofunderstanding the present disclosure, and should not be construed aslimiting the disclosure thereto.

Exemplary Embodiment

After a molten steel having a composition component of Table 1 was cast,the casted steel was reheated at 1100° C., the casted steel was wire-rodrolled to have a diameter of 15 mm, the wire rod was cooled to 300° C.,below a temperature, Bf, at a cooling rate of Table 2, and the wire rodwas air cooled, thereby manufacturing a wire rod. Meanwhile, Bf, abainite phase transformation end temperature, was measured using adilatometer, slightly varies depending on a chemical composition, andexists in the range of 300° C. to 350° C.

In the wire rod manufactured described above, a microstructure wasanalyzed and analysis thereof was illustrated in Table 2. In addition,tensile strength and impact toughness thereof were measured, andillustrated in Table 2. In a microstructure of the wire rod, an areafraction and a grain size of a martensite/austenite (M/A) constituentwere measured using an image analyzer, and a concentration of manganesewas measured using electron probe micro-analysis (EPMA).

In addition, a room temperature tensile test was carried out formeasurement, in which a crosshead speed was a rate of 0.9 mm/min to ayield point and was a rate of 6 mm/min thereafter. In addition, animpact test was carried out at room temperature for measurement, byusing an impact tester in which curvature of an edge portion of astriker for impacting a specimen was 2 mm and test capacity was 500 J.

TABLE 1 Rela- Rela- tional tional Expres- Expres- Composition component(weight %) sion sion No. C Si Mn Cr P S Ti B N Al 1 2 1 0.12 0.19 3.10.15 0.018 0.019 0.015 0.0025 0.0044 0.023 2.3 16.3 2 0.08 0.18 3.7 —0.017 0.020 0.020 0.0016 0.0049 0.015 12.6 20.6 3 0.10 0.13 3.6 0.180.014 0.017 0.017 0.0028 0.0042 0.040 7.7 27.7 4 0.07 0.20 3.4 0.070.011 0.015 0.025 0.0030 0.0036 0.033 24.1 17.0 5 0.11 0.18 3.5 0.240.016 0.013 0.030 0.0023 0.0039 0.038 39.0 19.4 6 0.05 0.16 3.8 0.220.015 0.015 0.011 0.0024 0.0044 0.043 −5.4 23.8 7 0.07 0.16 3.2 0.110.014 0.016 0.023 0.0017 0.0050 0.026 19.4 20.0 8 0.06 0.09 3 — 0.0130.011 0.027 0.0018 0.0048 0.020 31.3 33.3 9 0.10 0.15 3.9 0.10 0.0200.014 0.017 0.0027 0.0037 0.030 11.4 26.0 10 0.13 0.19 3.3 0.16 0.0160.018 0.013 0.0018 0.0045 0.035 −4.3 17.4 11 0.11 0.18 4 0.05 0.0090.020 0.019 0.0022 0.0040 0.044 15.4 22.2 12 0.25 0.16 3.4 — 0.014 0.0130.030 0.0025 0.0037 0.019 37.5 21.3 13 0.15 0.25 3.3 0.13 0.011 0.0150.021 0.0020 0.0050 0.022 12.1 13.2 14 0.11 0.15 2 0.07 0.018 0.0140.018 0.0005 0.0043 0.031 31.5 13.3 15 0.09 0.17 3.6 — 0.016 0.017 0.0210.0025 0.0041 0.028 16.9 21.2 16 0.08 0.16 3.2 0.21 0.011 0.016 0.020.0021 0.0047 0.017 11.7 20.0 17 0.06 0.15 3.5 0.17 0.012 0.011 0.0050.0027 0.0035 0.034 −9.9 23.3 18 0.07 0.18 4.3 0.12 0.010 0.012 0.0160.0018 0.0048 0.026 2.1 23.9 (In Table 1, Relational Expression 1 isMn + 5 (Ti − 3.5N)/B, Relational Expression 2 is Mn/Si, and a residualcomponent thereof is Fe and unavoidable impurities)

TABLE 2 Rela- Cool- M/A Impact tional ing M/A grain Tensile tough-Expres- Classifi- rate fraction size strength ness sion cation No. (°C./s) (%) (μm) (MPa) (J) 3 Inventive 1 0.5 7 3.9 659 158 2.1 example 2 18 3.3 660 163 2.6 3 0.2 5 4.7 652 180 2.3 4 2 10 2.0 680 159 2.4 5 1.3 92.4 664 160 2.2 6 1.9 9 2.1 670 152 2.8 7 1.5 8 2.3 665 168 2.3 8 0.3 54.6 635 199 2.0 9 0.8 7 3.5 657 172 2.7 10 0.7 7 3.8 650 155 2.2 11 1.18 3.3 663 165 2.9 Compara- 12 2 15 2.5 730 100 2.4 tive 13 1 11 3.5 75487 2.4 example 14 0.7 9 2.4 543 172 1.6 15 3 12 1.7 700 94 2.6 16 0.05 46.1 557 157 2.3 17 1 2 8.6 560 151 2.5 18 1.8 8 3.2 825 80 3.3 (In Table2, Relational Expression 3 is [Mn_(max)]/[Mn_(min)])

As illustrated in Tables 1 and 2, in the cases of Inventive examples 1to 11 satisfying a steel composition and a manufacturing method thereofaccording to the present disclosure, bainitic ferrite of 90 area % ormore may be obtained therefrom. For mechanical properties thereof, it isconfirmed that tensile strength of 600 MPa to 700 MPa and excellentimpact toughness of 150 J to 200 J were shown.

In the case of Inventive example 8, the content of silicon was 0.1 wt %or less, and thus, it is confirmed that impact toughness was furtherimproved. Among Inventive examples, in the cases of Inventive examples2, 3, 5, 7, 6, 9, and 11, satisfying Relational Expression 1(Mn+5(Ti−3.5N)/B 5.0) of manganese, titanium, boron, and nitrogen, inaddition to Relational Expression 2 (Mn/Si≧18) of manganese and silicon,it is confirmed that impact toughness was further excellent, as comparedto different cases.

In other words, among Inventive examples, in the cases of Inventiveexamples 1, 4, 6, and 10, not satisfying Relational Expression1(Mn+5(Ti−3.5N)/B≧5.0) and/or Relational Expression 2(Mn/Si≧18), it isconfirmed that impact toughness was somewhat reduced.

Meanwhile, in the case of Comparative example 12, the content of carbonwas higher. Thus, it is confirmed that tensile strength was excellent,but impact toughness was reduced. In this regard, because carbon wasdissolved in an M/A phase, a stable M/A phase was increased. In the caseof Comparative example 13, the content of silicon was outside of a rangeaccording to the present disclosure. In a manner similar to carbon, asan addition amount of silicon increases, an addition amount of siliconin a base increases. Thus, silicon has an effect of solid solutionstrengthening. In other words, when an addition amount of silicon wasabout 0.25%, while tensile strength may be significantly high, impacttoughness may be significantly reduced. In the case of Comparativeexample 14, since the hardenability of a steel material was reduced dueto an insignificant addition amount of manganese and boron, even when acooling condition was satisfied, ferrite and a bainitic ferritestructure were mixed, and thus, it is confirmed that tensile strengthwas reduced.

Meanwhile, in the case of Comparative example 15, a steel compositioncomponent thereof satisfies a range according to the present disclosure.As a cooling rate in a manufacturing process increases, martensite wasformed. Thus, it is confirmed that strength was increased, but impacttoughness was reduced. In the case of Comparative example 16, a steelcomposition component thereof satisfies a range according to the presentdisclosure, but a cooling rate in a manufacturing process was slow.Thus, it is confirmed that strength was reduced as ferrite was formed.

In addition, in the case of Comparative example 17, an addition amountof titanium was low. Since an amount of solute boron was reduced,hardenability was also reduced. When a cooling rate was low, aprecipitation amount of pro-eutectoid ferrite increases. Thus, it isconfirmed that tensile strength was reduced.

In addition, in the case of Comparative example 18, when a large amountof manganese was added thereto, relatively hardenability wassignificant. Even when cooling was performed at a cooling rate presentedin the present disclosure, martensite was formed. Thus, it is confirmedthat strength increases while impact toughness was reduced. In addition,since manganese was segregated in steel, due to formation of a locallyuneven structure, it is confirmed that impact toughness was reduced.

While the present disclosure has been particularly shown and describedwith reference to exemplary embodiments thereof, but is not limitedthereto. It will be apparent to those skilled in the art that variouschanges and modifications thereof may be made within the spirit andscope of the present disclosure, and therefore, it is to be understoodthat such changes and modifications belong to the scope of the appendedclaims.

1. A wire rod having enhanced strength and impact toughness comprising,by wt %, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less,manganese (Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur(S): 0.020% or less, boron (B): 0.0010% to 0.0030%, titanium (Ti):0.010% to 0.030%, nitrogen (N): 0.0050% or less, aluminum (Al): 0.010%to 0.050%, iron (Fe) as a residual component thereof, and otherunavoidable impurities, wherein a microstructure includes bainiticferrite in an area fraction of 90% or more, and a martensite/austenite(M/A) constituent as a residual component thereof.
 2. The wire rodhaving enhanced strength and impact toughness of claim 1, wherein thewire rod additionally comprises chromium (Cr): less than 0.3%.
 3. Thewire rod having enhanced strength and impact toughness of claim 1,wherein the content of manganese (Mn), titanium (Ti), boron (B), andnitrogen (N) satisfies Relational Expression 1,Mn+5(Ti−3.5N)/B≧5.0.   [Relational Expression 1]
 4. The wire rod havingenhanced strength and impact toughness of claim 1, wherein the contentof manganese (Mn) and silicon (Si) satisfies Relational Expression 2,Mn/Si≧18.   [Relational Expression 2]
 5. The wire rod having enhancedstrength and impact toughness of claim 1, wherein the wire rod isprovided with an arbitrary cross section in which a ratio of a maximumconcentration [Mn_(max)] and a minimum concentration [Mn_(min)] ofmanganese satisfies Relational Expression 3,[Mn_(max)]/[Mn_(min)]≦3.   [Relational Expression 3]
 6. The wire rodhaving enhanced strength and impact toughness of claim 1, wherein agrain size of the martensite/austenite (M/A) constituent is 5 μm orless.
 7. A method of preparing a wire rod having enhanced strength andimpact toughness comprising: reheating a steel material including, by wt%, carbon (C): 0.05% to 0.15%, silicon (Si): 0.2% or less, manganese(Mn): 3.0% to 4.0%, phosphorus (P): 0.020% or less, sulfur (S): 0.020%or less, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.010% to 0.030%,nitrogen (N): 0.0050% or less, aluminum (Al): 0.010% to 0.050%, iron(Fe) as a residual component thereof, and other unavoidable impurities;hot rolling the steel material having been reheated; cooling the steelmaterial at a rate of 0.1° C./s to 2° C./s in a temperature within arange of Bf° C. to Bf−50° C., after the hot rolling; and air cooling thesteel material having been cooled.
 8. The method of preparing a wire rodhaving enhanced strength and impact toughness of claim 7, wherein thesteel material additionally includes chromium (Cr): less than 0.3%. 9.The method of preparing a wire rod having enhanced strength and impacttoughness of claim 7, wherein the content of manganese (Mn), titanium(Ti), boron (B), and nitrogen (N) satisfies Relational Expression 1,Mn+5(Ti−3.5N)/B≧5.0.   [Relational Expression 1]
 10. The method ofpreparing a wire rod having enhanced strength and impact toughness ofclaim 7, wherein the content of manganese (Mn) and silicon (Si)satisfies Relational Expression 2,Mn/Si≧18.   [Relational Expression 2]
 11. The method of preparing a wirerod having enhanced strength and impact toughness of claim 7, whereinthe reheating is performed at a temperature of 1000° C. to 1100° C. 12.The method of preparing a wire rod having enhanced strength and impacttoughness of claim 7, wherein finish hot rolling, of the hot rolling, isperformed at a temperature within a range of 850° C. to 950° C.